1. Field of the Invention
The present invention relates to a touch panel and a display apparatus having the same. More particularly, the present invention relates to a touch panel having a liquid crystal layer and a display apparatus having the touch panel.
2. Description of the Related Art
A touch panel is an electric device that executes an internal program in response to a touch event such as a compression, light, etc. The touch panel receives the touch event from the exterior and detects a touch position of the touch event, and then executes the internal program corresponding to the touch position.
The touch panel is adapted to a flat display device such as a liquid crystal display device (“LCD”). The LCD having the touch panel detects the touch position of the touch event applied on a display screen thereof, and executes the internal program corresponding to the touch position to display an image.
The touch panel includes sensor lines, sensor switches, a line selecting circuit and an operational amplifier. Input terminals of the sensor switches are electrically connected to a common electrode to which a common voltage is applied. Output terminals of the sensor switches are electrically connected to an input terminal of the line selecting circuit via the sensor lines. An output terminal of the line selecting circuit is electrically connected to the input terminal of the line selecting circuit so that the output terminal of the line selecting circuit sequentially outputs sensing signals transferred via the sensor lines. The operational amplifier receives the sensing signals from the line selecting circuit and compares the sensing signals with a reference voltage so that the operational amplifier generates a sensing output signal having information regarding whether the touch event has occurred.
An input terminal of the operational amplifier is electrically connected to a pull-up resistor so that the operational amplifier compares the sensing signals with the reference voltage. That is, a first end of the pull-up resistor receives a driving direct current (“DC”) voltage, and a second end of the pull-up resistor opposite to the first end of the pull-up resistor is electrically connected to the input terminal of the operational amplifier.
The input terminal of the operational amplifier is electrically connected to the sensor switch via the line selecting circuit so that the input terminal of the operational amplifier receives the voltage of an output terminal of the sensor switch during turn-on time of the line selecting circuit. Therefore, the operational amplifier compares the voltage of an output terminal of the sensor switch with the reference voltage.
Since the input terminal of the operational amplifier is electrically connected to the line selecting circuit and receives the driving DC voltage through the pull-up resistor, voltage levels of the sensing signals applied to the input terminal of the operational amplifier may be changed whenever the line selecting circuit sequentially receives the sensing signals from the sensor lines.
The input terminal of the operational amplifier receives the driving DC voltage through the pull-up resistor when the touch event does not occur. On the other hand, when the touch event occurs, the input terminal of the operational amplifier receives the common voltage through the sensor line and the line selecting circuit from the output terminal of the sensor switch.
The sensor line is formed on the touch panel to have a high parasitic capacitance so that the sensor line stores some quantity of charge. The sensor line receives the driving DC voltage through the pull-up resistor and the line selecting circuit during the turn-on time of the line selecting circuit. A large amount of time may be required to charge the output terminal of the sensor switch with the driving DC voltage due to the high parasitic capacitance of the sensor line. Thus, the sensor line electrically connected to the output terminal of the sensor switch may not be charged with the driving DC voltage. Also, the pull-up resistor and the turn-on resistor of the line selecting circuit may prevent the output terminal of the sensor switch from being charged with the driving DC voltage.
When the output terminal of the sensor switch and the sensor line are charged with a voltage different from the driving DC voltage, the touch panel may execute the internal program of the touch event even though a touch event does not occur during the turn-on time of the line selecting circuit.